1. Field of the Invention
This invention relates to a method of producing steel having a low yield ratio.
2. Description of the Prior Art
In recent years, in various fields such as the shipbuilding industry and the industrial machinery industry there is an increasing demand for steels that enable welding operations to be reduced and properties such as bendability to be pursued to the limit, have better weldability and which will enable steel costs to be lowered.
Of these, in order to improve the bendability of steel plate it is necessary to develop plate that possesses a low yield ratio. Low-yield-ratio steel is also desirable for improving the safety of structures such as buildings and bridges, especially the earthquake resistance of such structures.
In conventional controlled rolling, controlled cooling process, to achieve improved low-temparature toughness, in the hot-rolling the ferrite grains are made as small as possible and accelerated cooling from the austenitic single phase is employed.
However, a problem with this method is that the yield point rises due to the refinement of the ferrite grains, the hardening and the formation of part of the pearlite into bainite, resulting in a higher yield ratio that reduces the bendability.
In methods for lowering the yield point using a controlled rolling, controlled cooling process, there has also been proposed a method of producing steel having a low yield ratio whereby a low yield point is achieved together with good low-temperature toughness provided by a fine-grain ferritic structure. However, the need for still lower yield ratios has continued to grow.
JP-B-No. 56(1971)-4608 proposes low-temperature toughness steel containing 4.0 to 10% nickel for use as a material for liquid natural gas containers.